bundle update --conservative changes shared dependencies when updating just one group

bundle env output

## Environment

```
Bundler             1.16.2
  Platforms         ruby, x86_64-linux
Ruby                2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
  Full Path         /home/eero/.rvm/rubies/ruby-2.5.1/bin/ruby
  Config Dir        /home/eero/.rvm/rubies/ruby-2.5.1/etc
RubyGems            2.7.6
  Gem Home          /home/eero/.rvm/gems/ruby-2.5.1
  Gem Path          /home/eero/.rvm/gems/ruby-2.5.1:/home/eero/.rvm/gems/ruby-2.5.1@global
  User Path         /home/eero/.gem/ruby/2.5.0
  Bin Dir           /home/eero/.rvm/gems/ruby-2.5.1/bin
Tools               
  Git               2.17.0
  RVM               1.29.3 (latest)
  rbenv             not installed
  chruby            not installed
  rubygems-bundler  (1.4.4)
```

## Bundler Build Metadata

```
Built At          2018-05-16
Git SHA           9f7bf0ac3
Released Version  true
```

## Gemfile

### Gemfile

```ruby
source 'https://rubygems.org'

gem 'rake'

group :test do
  gem 'capistrano'
end
```

### Gemfile.lock

```
GEM
  remote: https://rubygems.org/
  specs:
    airbrussh (1.3.0)
      sshkit (>= 1.6.1, != 1.7.0)
    capistrano (3.10.1)
      airbrussh (>= 1.0.0)
      i18n
      rake (>= 10.0.0)
      sshkit (>= 1.9.0)
    concurrent-ruby (1.0.5)
    i18n (0.9.5)
      concurrent-ruby (~> 1.0)
    net-scp (1.2.1)
      net-ssh (>= 2.6.5)
    net-ssh (4.2.0)
    rake (12.3.0)
    sshkit (1.16.1)
      net-scp (>= 1.1.2)
      net-ssh (>= 2.8.0)

PLATFORMS
  ruby

DEPENDENCIES
  capistrano
  rake

BUNDLED WITH
   1.16.2
```